1. Field of the Invention
The present invention relates to a sound signal processing method and a sound reproduction apparatus, which are useful when listening to sounds with headphones or earphones and localizing a sound image at an arbitrary fixed position outside the head of a listener, or when listening to sounds with speakers or headphones and localizing a sound image at an arbitrary changeable position around the listener.
2. Description of the Related Art
A sound reproduction system is proposed in which, when listening to sounds with headphones, a sound image is localized at an arbitrary fixed position outside the head of a listener regardless of which direction the listener faces, as if a speaker is disposed at the fixed position.
FIGS. 1A, 1B and 1C show the principle for such sound image localization. As shown in FIG. 1A, a listener 1 wears headphones 3 and listens to sounds with left and right acoustic transducers 3L, 3R of the headphones 3. Then, as shown in FIG. 1B or 1C, a sound image is localized at an arbitrary fixed position, which is denoted by a sound source 5, outside the listener's head regardless of whether the listener 1 faces rightward or leftward.
In that case, it is assumed that HL and HR represent respective Head Related Transfer Functions (HRTF) from the sound source 5 to a left ear 1L and a right ear 1R of the listener 1, and HLc and HRc represent, in particular, respective Head Related Transfer Functions from the sound source 5 to the left ear 1L and the right ear 1R of the listener 1 when the listener 1 faces in a predetermined direction, e.g., in a direction toward the sound source 5. In the following description, the facing direction of the listener 1 is represented by a rotational angle θ with respect to the direction toward the sound source 5.
FIG. 17 shows one example of conventional sound reproduction systems implementing the above-described principle. An angular velocity sensor 9 is attached to the headphones 3, and an output signal of the angular velocity sensor 9 is integrated to detect the rotational angle θ.
In the example of FIG. 17, an input digital sound signal Di corresponding to a signal from the sound source 5 in FIG. 1 is supplied to digital filters 31 and 32. The digital filters 31 and 32 convolute impulse responses corresponding to the Transfer Functions HLc and HRc on the digital sound signal Di, and are constituted as, e.g., FIR (Finite Impulse Response) filters.
Sound signals L1 and R1 outputted from the digital filters 31 and 32 are supplied to a time difference setting circuit 38. Then, sound signals L2 and R2 outputted from the time difference setting circuit 38 are supplied to a level difference setting circuit 39.
When the listener 1 faces rightward as shown in FIG. 1B, the left ear 1L of the listener 1 comes closer to the sound source 5 and the right ear 1R moves farther away from the sound source 5 as the rotational angle θ increases within the range of θ=0 degree to +90 degrees. To fixedly localize a sound image at the position of the sound source 5, therefore, the Transfer Function HL must be changed relative to the Transfer Function HLc such that as the rotational angle θ increases, a resulting time delay is reduced and an output signal level is increased, while the Transfer Function HR must be changed relative to the Transfer Function HRc such that as the rotational angle θ increases, a resulting time delay is increased and an output signal level is reduced.
Conversely, when the listener 1 faces leftward as shown in FIG. 1C, the left ear 1L of the listener 1 moves farther away from the sound source 5 and the right ear 1R comes closer to the sound source 5 as the rotational angle θ increases within the range of θ=0 degree to −90 degrees. To fixedly localize a sound image at the position of the sound source 5, therefore, the Transfer Function HL must be changed relative to the Transfer Function HLc such that as the rotational angle θ increases, a resulting time delay is increased and an output signal level is reduced, while the Transfer Function HR must be changed relative to the Transfer Function HRc such that as the rotational angle θ increases, a resulting time delay is reduced and an output signal level is increased.
In the sound reproduction system of FIG. 17, the time difference between the sound signal listened by the listener's left ear and the sound signal listened by the listener's right ear is set by the time difference setting circuit 38, and the level difference between them is set by the level difference setting circuit 39.
More specifically, the time difference setting circuit 38 comprises time delay setting circuits 51 and 52. In the time delay setting circuits 51 and 52, the sound signals L1 and R1 outputted from the digital filters 31 and 32 are successively delayed by multistage-connected delay circuits 53 and 54. The delay circuits 53 and 54 serve as delay units each providing a delay time for each stage, which is equal to a sampling period τ of the sound signals L1 and R1.
For example, sampling frequency fs of the sound signals L1 and R1 is 44.1 kHz, and therefore the sampling period τ of the sound signals L1 and R1 is about 22.7 μsec. This value corresponds to a change in time delay of the left and right sound signals occurred when the rotational angle of the listener's head is about 3 degrees.
In the time delay setting circuits 51 and 52, output signals from stages of the delay circuits, which correspond to a rotational angle (direction) closest to the detected rotational angle θ, are taken out by respective selectors 55 and 56 as the sound signals L2 and R2 outputted from the time difference setting circuit 38.
For example, when the rotational angle θ is 0 degree, output signals Lt and Rt at the middle stages of the delay circuits are taken out by the selectors 55 and 56, and the time difference between the output sound signals L2 and R2 becomes 0. When the rotational angle θ is +α (i.e., α in the rightward direction, α being about 3 degrees corresponding to τ), a signal Ls advanced τ from the signal Lt is taken out by the selector 55 and a signal Ru delayed τ from the signal Rt is taken out by the selector 56. When the rotational angle θ is −α (i.e., α in the leftward direction), a signal Lu delayed τ from the signal Lt is taken out by the selector 55 and a signal Rs advanced τ from the signal Rt is taken out by the selector 56.
In the level difference setting circuit 39, respective levels of the sound signals L2 and R2 outputted from the time difference setting circuit 38 are set depending on the detected rotational angle θ, whereby the level difference between the sound signals L2 and R2 is set.
Then, digital sound signals L3 and R3 outputted from the level difference setting circuit 39 are converted to analog sound signals by D/A (Digital-to-Analog) converters 41L and 41R. The resulting 2-channel analog sound signals are amplified by sound amplifiers 42L and 42R, and supplied to the left and right acoustic transducers 3L, 3R of the headphones 3, respectively.
FIG. 18 shows another example of the conventional sound reproduction systems. In this example, digital filters 83-0, 83-1, 83-2, . . . , 83-n and digital filters 84-0, 84-1, 84-2, . . . , 84-n are provided to convolute, on an input digital sound signal, impulse responses corresponding to Head Related Transfer Functions HL(θ0), HL(θ1), HL(θ2), . . . , HL(θn) from the sound source 5 to the left ear 1L of the listener 1 in FIG. 1 and Head Related Transfer Functions HR(θ0), HR(θ1), HR(θ2), . . . , HR(θn) from the sound source 5 to the right ear 1R of the listener 1, when the rotational angle θ is θ0, θ1, θ2, . . . , θn, respectively. The rotational angles θ0, θ1, θ2, . . . , θn are set at, for example, equiangular intervals in the circumferential direction about the listener.
Then, an input digital sound signal Di is supplied to the digital filters 83-0, 83-1, 83-2, . . . , 83-n and the digital filters 84-0, 84-1, 84-2, . . . , 84-n. An output signal from one of the digital filters 83-0, 83-1, 83-2, . . . , 83-n, which corresponds to a rotational angle (direction) closest to the detected rotational angle θ, is taken out by a selector 55 as a sound signal to be supplied to the left acoustic transducer 3L of the headphones 3. An output signal from one of the digital filters 84-0, 84-1, 84-2, . . . , 84-n, which corresponds to a rotational angle (direction) closest to the detected rotational angle θ, is taken out by a selector 56 as a sound signal to be supplied to the right acoustic transducer 3R of the headphones 3.
Then, digital sound signals outputted from the selectors 55 and 56 are converted to analog sound signals by D/A converters 41L and 41R. The resulting 2-channel analog sound signals are amplified by sound amplifiers 42L and 42R, and supplied to the left and right acoustic transducers 3L, 3R of the headphones 3, respectively.
In the conventional sound reproduction system shown in FIG. 17, however, the resolution of a time delay in the Head Related Transfer Functions (HRTF) HL and HR from the sound source 5 to the left ear 1L and the right ear 1R of the listener 1 in FIG. 1 is decided by the unit delay time of the delay circuits 53 and 54 in the time delay setting circuits 51 and 52, i.e., by the sampling period τ of the sound signals L1 and R1 outputted from the digital filters 31 and 32. Hence, when the sampling frequency fs of the sound signals L1 and R1 is 44.1 kHz and the sampling period τ is about 22.7 μsec, the resolution of the time delay corresponds to about 3 degrees in terms of the rotational angle of the listener's head.
Therefore, when the facing direction of the listener is not a discrete predetermined direction represented by 0 degree or an integral multiple of ±3 degrees that is decided by the sampling period τ of the sound signals L1 and R1 outputted from the digital filters 31 and 32, but a direction between the discrete predetermined directions, such as ±1.5 or ±4.5 degrees, a sound image cannot be localized at the predetermined position (direction), denoted by the sound source 5 in FIG. 1, precisely corresponding to the facing direction of the listener.
Also, when the listener changes the facing direction, the sound signals L2 and R2 outputted from the time difference setting circuit 38 are momentarily changed over for each unit angle. Hence, waveforms of the sound signals L2 and R2 are changed abruptly and transfer characteristics are also changed abruptly, whereby shock noises are generated.
Similarly, in the conventional sound reproduction system shown in FIG. 18, when the facing direction of the listener is not a discrete predetermined direction, but a direction between the discrete predetermined directions, such as between θ0 and θ1 or between θ1 and θ2, a sound image cannot be localized at the predetermined position (direction) denoted by the sound source 5 in FIG. 1 precisely corresponding to the facing direction of the listener. Also, when the listener changes the facing direction, the sound signals outputted from the selectors 55 and 56 are momentarily changed over for each unit angle. Hence, waveforms of the output sound signals are changed abruptly and transfer characteristics are changed abruptly, whereby shock noises are generated.